Since losing their first two games in the 24 hours after seeing All-Star newcomer Gordon Hayward suffer a horrific ankle dislocation and fractured tibia on opening night, the Boston Celtics have ripped off 12 consecutive wins to hold the NBA's best record at 12-2.

Despite more bad luck in terms of injuries with stars Al Horford (concussion) and Kyrie Irving (facial fracture) missing time, Boston has relied on the league's best defense and a series of timely comebacks to keep on winning.

While the 70-win season the Celtics are currently on pace for isn't realistic, can they sustain enough of their hot start to remain championship contenders without Hayward?

How Boston is winning

Without Hayward, and particularly without either Horford or Irving on top of that, the Celtics have been below-average offensively. They rank 18th in offensive rating, and even that has been bolstered by frequent second-chance scores and efficiency in transition. According to CleaningTheGlass.com, Boston's half-court offense ranks 23rd in terms of points per play. Of the seven teams averaging fewer points per play in the half court, only the Denver Nuggets are .500 or better.

The Celtics' fast start, then, has been built almost entirely at the defensive end. Boston's league-leading 95.4 points allowed per 100 possessions is 3.1 points per 100 better than the next-best team in defensive rating (the Oklahoma City Thunder) and 8.4 percent better than league average (104.1 points per 100 possessions). If the Celtics were able to maintain that, it would surpass their 2007-08 championship team as the second-best defense relative to league average since the ABA-NBA merger.

Boston's record defense

Team

Season

Rating

Average

Difference

San Antonio

2003-04

91.6

100.0

8.5

Boston

2017-18

95.4

104.1

8.4

Boston

2007-08

96.2

104.7

8.2

New York

1992-93

96.7

105.0

7.9

New York

1993-94

95.2

103.3

7.9

Of course, the odds are Boston won't continue to play this well defensively. There are plenty of examples of teams starting a season at historic levels only to regress to the mean. And the Celtics' reliance on holding opponents to low shooting percentages makes them particularly vulnerable.

Will opponents keep missing shots?

As Ben Falk investigated on CleaningTheGlass.com last month, opponent shooting percentages early in the season tend to be less predictive than shot distribution, which hasn't been a real strength for Boston. If all opponents were making shots at a league-average rate from each of the zones tracked by NBA.com (in the restricted area, other paint attempts, non-paint 2-pointers, corner 3-pointers and above-the-break 3-pointers), the Celtics' defensive shot distribution would put them 12th in the league in opponent effective field-goal percentage (eFG). They actually lead the league in this category. (Intriguingly, Boston actually had better shot distribution last season, ranking seventh in expected eFG. The Celtics' actual eFG allowed ranked 13th.)

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Boston's defense does look more sustainable when we consider how well they've contested opponent shots. Based on Second Spectrum analysis, the Celtics have the best defensive quantified shot quality (qSQ) in the league at 49.9 percent. That measures the eFG we'd expect based not only on the location of shot attempts, but also of nearby defenders (plus shot type), providing more detail. Still, Boston also leads the league in quantified shooter impact (qSI), which is the difference between the expected qSQ and actual eFG (47.7 percent) -- meaning teams are making far fewer shots against the Celtics than we'd expect after accounting for their contests.

That's particularly true of 3-point shots. The quality of the 3s Boston has allowed is almost exactly average as measured by qSQ, which suggests opponents should be making them at a 35.6 percent clip. Instead, Celtics opponents are actually making just 32.1 percent of their 3s, the league's third-lowest mark. Falk found that the 3-point percentage a team allows over the first seven games has no relationship whatsoever to the 3-point percentage they allow the remainder of the season, and while the sample is slightly larger now, the same trend holds.

Fans of Boston will protest that the team has a history of holding opponents to below-average 3-point shooting, and that's true. The Celtics have ranked in the top five in opponent 3-point percentage all four seasons Brad Stevens has coached, and were second in the NBA in 2016-17. While this does look like more than good luck, it's worth noting the effect is limited. Opponents have shot about 95 percent of league average from 3 against Boston during Stevens' tenure, as compared to 89 percent so far this season. So some regression should be expected.

Will the Celtics keep winning close games?

As well as Boston has played defensively, it's nonetheless surprising the Celtics haven't dropped a game somewhere over the past three weeks. Boston trailed by 18 points in two games during its winning streak, at Oklahoma City and at home against Charlotte, only to rally both times. And including the Hornets game, three of the Celtics' past four wins have come by three points or fewer.

Boston hasn't actually been exceptional in clutch situations. The Celtics have outscored opponents by 15.5 points per 100 possessions in these situations, according to NBA.com/Stats, which ranks eighth in the league. Boston is third in rating differential overall and had the fifth-best clutch differential (plus-14.9 points per 100 possessions) last season.

Individually, Irving has been exceptional. He's finishing 49.1 percent of the Celtics' plays in clutch situations, which ranks second to DeMar DeRozan among qualified players, yet has posted an impressive .619 true shooting percentage. Irving has largely replaced Isaiah Thomas' production in the clutch last season, when Thomas had a 46.0 percent usage rate and a .654 true shooting percentage.

Boston did lose a pair of close games to start the season. Still, it's not realistic to expect the Celtics to keep winning nearly 80 percent of their games decided by 10 points or fewer (7-2). Add in some regression defensively and Boston is probably playing more like a 55-win team than a 70-win one. That would almost certainly be good enough for the Celtics to finish first in the Eastern Conference for the second season in a row, particularly given they've already banked their 12 wins so far.

Such a pace is around what's suggested by FiveThirtyEight's Carm-ELO projections (which have Boston finishing with an average of 56 wins) and ESPN's Basketball Power Index (an average of 60 wins). The Celtics finish with the top record in the East in 93 percent of BPI's simulations, nearly as frequently as the Golden State Warriors claim the top seed in the West (94 percent).

Boston found out the hard way last season that being the No. 1 seed doesn't necessarily mean anything against LeBron James in the playoffs. So whether the Celtics can get to the NBA Finals probably has more to do with whether the Cleveland Cavaliers can solve their defensive woes than how much Boston comes back to Earth. Nonetheless, that we can even pose the question of whether the Celtics are championship contenders without one of their marquee offseason additions is impressive testament to how they've played defensively.